dashiichi: To the best of my understanding:

1.a. There are no current user-side differences. The back end code (networking, etc.) might have some differences.Once upon a time, the PC multiplayer engine required a Steam API, such that "in person" multiplayer on PCs was only functional via Steam. That has since been resolved and multiplayer works across all platforms.

1.b. Updates of the PC version show up first on Steam. The delay to appearing on GoG has ranged from several minutes to over a weekend. Typically it's an hour or two.

2. Verions on both platforms are in sync; they both get the same content.

3. Yes. Disclaimer: I run single player except for rare scheduled GoG meet-ups. AIUI, all platforms (consoles + PC) can now meet and mingle in multiplayer.

4. The multiplayer thing, back when it first came out. Multiplayer was fenced by platform and, on the PC, relied on the Steam API. Since resolved.

Additional. At least through the current release, the multiplayer defaults allow other players to damage your ship, your base, and you. And there are griefers out there. And they can use the save editor to juice their weapons and shields. Recommend going into the network options and setting "can damage" to nobody, at lease initially.

scrubjay09: I prefer GOG.
1) You own the game vs rent the game on Steam. If Steam has issues you may not be able to play the game you purchased.
2) You control the updates. There have been many games on Steam (like ARK) where the game has been broken by patches which are forced on you by Steam. In GOG, you can control the patches, Steam you can't.

Tatwi: This is why I bought the game on GOG again after initially playing it on Steam. For a time, I prefered to play version 1.77 and also for a time versions newer than 1.77 ran very poorly with my video card. Being able to re-install with the old GOG installer was handy (though it's only possible if you have previously downloaded the offline installer and kept it around, as GOG doesn't keep all the older versions available for download).

That said, anyone who is already heavily invest in Steam would likely do fine buying it on Steam. They're likely going to be around longer than we'll all live anyway. :)